As a step to analyze the potential carrying capacity of symbionts in the forest, the potential ability for propagation of ectomyccorrhizal fungi in a urea treated area was examined. Urea was amended on the floor of a Quercus mongolica - Betula plathylla dominant mixed forest at Sugadaira or on that of a Quercus glauca - Abies firma - Pinus densiflora dominant mixed forest at Kiyosumi. The appearance frequency of ectomycrrhizal fungi increased but their flora became more simple, such as Hebeloma spoliatum and Calocybe leucocephala at Sugadaira, and H.Spoliatum and Laccaria bicolor at Kiyosumi, campared with that on non-urea-treated floor (=control) where more than 7 species of ectomycrrhizal fungi occurred. In the H-A horizon soil where development of mycorrhizae was observed, the biomass of soil microfungi, total bacteria, and crystal-violet-resistant bacteria decreased during the development of myccorrhzae of H.spoliatum. The total dry weight of the fruit bodies of ectomycorrhizal fungi occurring on the urea-treated floor at Kiyosumi in the first year was about 30.5 g/dry soil/year, and this value was about 95 times larger than that obtained in the control. The productivity of fruit bodies of ectomycorrhizal fungi on the urea-treated floor decreased in the second year, although it was about twofold larger than that in the control. These findings suggest that the carrying capacity of the symbionts which propagate under weak interactions among soil microbes, in the forest was at least 95 times larger than that of the symbionts propagaing under a normal condition, namely with strong interactios among soil microbes.